1. Field of the Invention
The present invention relates generally to electrical discharge machining and, more specifically, to a method and apparatus for preparing a surface of a cylinder bore, such as in an engine, brake caliper or other component, by electrical discharge machining.
2. Description of the Related Art
It is known to employ a plurality of pistons which reciprocate within cylinder bores of an engine under forces created by combustion of an air and fuel mixture therein. It is also known that the pistons, piston rings and cylinder bores wear relative to each other during this reciprocation. As a result, cast iron wear sleeves or inserts have been employed within the cylinder bores between the pistons, piston rings and a surface of the cylinder bores to improve the wear characteristics at this juncture.
Although the above cast iron wear sleeves have worked well, they suffer from the disadvantage that cast iron is relatively heavy, and therefore adds weight and cost to the engine. In order to eliminate these disadvantages in automotive engines, thermal spray or wear coatings have been developed to take the place of the cast iron wear sleeves while retaining the wear characteristics of the piston/cylinder bore arrangement.
Typically, the surface of the cylinder bore of an automotive engine is very smooth and must be prepared or toughened in order that the thermal wear coating can adhere thereto. A number of methods and apparatuses have been employed to prepare the surface of the cylinder bore. However, all have certain disadvantages.
For example, it is known to prepare the surface of the cylinder bore using conventional machine tools and methods. However, this has been found to suffer from the disadvantage that adequate surface preparation is typically not achieved and it is difficult to position the machine tool relative to the bore.
It is also known to employ grit blasting to produce many fine pock marks or minutely roughen the surface of the cylinder bore. Unfortunately, grit blasting suffers from the disadvantage that it is a relatively messy operation wherein grit, dust and other particles are generated in the work area. Thus, while relatively effective as a surface preparation, grit blasting has resulted in other, less desirable, disadvantages which overall reduces the effectiveness of grit blasting as a method for preparing the surface of cylinder bore for a thermal wear coating.
It is further known to employ water or H.sub.2 O blasting similar to grit blasting but wherein water is employed at very high pressure to prepare the surface of the cylinder bore. However, water blasting suffers from the disadvantage that it requires very expensive equipment operating at very high pressures such that it is not cost competitive. Further, the water may introduce a corrosion factor into the equation further limiting the application of water blasting for this purpose.